This paper proposes the economic and managerial rationale and technical feasibility of implementing computer monitoring of power plant auxiliary systems, and puts forward a technical solution based on the current status of power plant auxiliary control systems. 1 Introduction With the further deepening of power system reform, ensuring high-quality and efficient equipment operation, improving labor productivity, and further enhancing economic benefits have become the goals of power plant development. The automated control of thermal power plant auxiliary systems is an important component of the overall thermal control of the plant, and its automation level directly affects the overall automation level of the plant. Therefore, while upgrading main equipment and transforming unit control systems using DCS, power plants have also strengthened the transformation of auxiliary system control systems using PLC technology, and the automation level of newly built unit auxiliary systems has been greatly improved accordingly. However, overall, the control systems of various auxiliary workshops are not interconnected, and there is little communication with the DCS, forming multiple automation islands, causing inconvenience in operation and management, serious waste of personnel, low production efficiency, and hindering the further improvement of the overall plant automation level. With the development of computer control technology and the maturity of communication and network technologies, connecting the various auxiliary workshops of thermal power plants into one and realizing centralized monitoring of auxiliary systems has become a achievable reality. The main points raised at the "Symposium on Several Issues of Power Plant Automation" held by the Electric Power Planning Institute in 2002 also pointed out: "With the continuous improvement of the automation level of auxiliary systems, most newly built power plants now consider the centralized control network of auxiliary system PLCs. To ensure the normal operation of the centralized control network of auxiliary system PLCs, a unified wiring plan should be made for the centralized control networks of water, ash, and coal PLCs, as well as the centralized control network of the entire plant's auxiliary systems, during the preliminary design stage, and a corresponding wiring plan diagram for the centralized control network of auxiliary system PLCs should be proposed." According to a survey conducted by the State Power Corporation's research group on "Research on the Improvement of Thermal Automation Technology in Thermal Power Plants," the realization of networking and centralized control of auxiliary workshop monitoring systems in thermal power plants remains a long and arduous task. Of the 168 thermal power plants with a capacity of 125MW or above surveyed, only 21 have achieved networking to varying degrees. Centralized control of many auxiliary workshops in a large number of power plants has not yet been implemented, and the excessive number of personnel operating in auxiliary workshops seriously affects the realization of personnel reduction and profit enhancement in these power plants. It is expected that with the gradual completion of the DCS transformation of unit units, the automation transformation of auxiliary workshops will soon be fully implemented. 2 Characteristics of Auxiliary Systems in Thermal Power Plants Auxiliary systems in thermal power plants include coal conveying systems, fuel oil pumping stations, ash and slag removal systems, chemical water treatment systems, condensate treatment systems, wastewater treatment systems, circulating water systems, and boiler soot blowing systems. Their normal operation is an important guarantee for the safe and economical operation of the unit and even the entire power plant. They mainly have the following characteristics: (1) Dispersed locations; except for the boiler soot blowing system, the coal conveying system, fuel oil pumping station, ash and slag removal system, chemical water treatment system, condensate treatment system, wastewater treatment system, and circulating water system are distributed throughout the plant, and almost all of them have corresponding control rooms and are equipped with corresponding operating personnel. (2) Non-continuous control; except for the circulating water system, the other systems are almost all intermittently operated, that is, they are operated only after certain requirements are met, and the system stops operating after certain conditions are met, waiting for the next operation. (3) The main control room is required to keep track of the status of the auxiliary systems at all times to ensure the normal operation of the entire power plant. (4) Each system should be in a healthy state, and problems should be dealt with in a timely manner, otherwise it will affect the safe and economical operation of the entire plant. 3 Current Status and Problems of Control The current status of auxiliary control systems can be roughly summarized as follows: (1) The core of most major auxiliary control systems adopts a control system composed of PLC and host computer. Some minor auxiliary control systems are also gradually adopting small PLC for control. Those that do not adopt PLC use conventional control devices such as local manual operation, remote operation and combination instruments. (2) The integration of auxiliary control systems is relatively decentralized and is often supplied by different manufacturers, resulting in different types of PLCs in each control system. (3) Switch control occupies the core of auxiliary system control. A large number of valves, solenoid valves, motors, etc. are subject to the logic control of interlocking conditions. The control is mainly based on the equipment status, valve opening and closing, motor start and stop, pressure flow and material level limits. (4) Major auxiliary systems such as coal conveying system, fuel pump room, ash removal and slag removal system, chemical water treatment system, circulating water system, etc. need to provide a dedicated control room and be equipped with a certain number of operators for on-duty operation. (5) Each control system is independent of each other. There is almost no exchange of system information with the main control room and between them, let alone information exchange with MIS. Therefore, the current control status of the auxiliary system brings about the following problems: (1) The decentralized control room not only increases investment and operation and maintenance costs, such as decoration and air conditioning, but is also difficult to manage. (2) PLC control results in less workload for on-duty personnel, but the overall number of personnel is too large. For example, there are 5 main control personnel per unit in a certain plant, but about 15 on-duty personnel for peripheral equipment. (3) There is very little automatic information exchange between the auxiliary systems and the main control room. The main control room often communicates the control requirements of the auxiliary systems by telephone, which makes auxiliary control inconvenient and untimely. (4) Different hardware and software are used in each auxiliary control system, which makes spare parts management, personnel training and maintenance difficult. (5) Important information of each auxiliary system cannot be connected to the MIS network. 4 Discussion of technical solutions With the deepening of power reform, the impact of the above problems on the overall economic efficiency of the plant will become more and more obvious. Therefore, power plants at home and abroad are paying more and more attention to solving these problems. At the same time, the development of computer control technology, computer network technology and computer software technology has also created sufficient conditions for solving these problems. 4.1 Overall Structure Several schemes can be chosen to achieve centralized computer monitoring of the auxiliary system. Centralized control can be adopted, whereby the signals of the auxiliary systems are connected to the DCS system in the main control room for centralized monitoring. Alternatively, a relatively centralized approach can be used, establishing several relatively centralized control points. Auxiliary systems that are physically close or closely connected are first centralized at these control points for centralized monitoring, and then these sub-networks are connected to the DCS and MIS networks. Based on the current characteristics of auxiliary system control in my country's thermal power plants, the entire auxiliary control system can be divided into three control networks: coal, ash, and water. The coal network can include coal conveying control and related auxiliary systems, and fuel oil pumping stations; the ash network can include ash and slag removal systems, dry ash conveying systems, and electrostatic precipitator control systems; the water network can include chemical water treatment, condensate polishing, steam and water sampling and analysis, chemical dosing systems, water purification stations, chemical and domestic wastewater treatment, ash water treatment, and circulating water chemical dosing systems. The corresponding system's control cabinets are located in the auxiliary system's electronics room or near the equipment. The host computer operator stations are located in the local centralized control room at the three control points, enabling operation monitoring. The control system communicates with the DCS and MIS via a network server, allowing monitoring of the auxiliary systems from the monitoring terminal in the main control room. The hardware for centralized computer monitoring of the auxiliary systems can be selected from DCS, PLC, host computer, or fieldbus technology, depending on the specific circumstances. This forms the basis of a comprehensive monitoring system for the power plant's auxiliary workshop. This auxiliary workshop control system scheme can be divided into four layers: the field layer (physical layer), the control layer, the monitoring layer, and the management layer. The field layer includes field I/O stations and other control interface devices, processing field signals and outputting control signals. The control layer implements the control programs for each auxiliary system and provides auxiliary operation functions. The monitoring layer implements separate monitoring of the auxiliary workshops according to the water, coal, and ash process systems, including process monitoring, control operations, system maintenance, etc., and includes operator stations and engineer stations. The management layer integrates information from the production workshop (including the DCS and auxiliary workshops), provides connectivity to the plant-wide network, coordinates the operation between auxiliary workshops and the main control room DCS, and manages daily tasks such as work order management. All layers are connected via a communication network, forming a complete control system. The four layers are interconnected through a communication network, with three types of communication: communication between field I/O stations and the control layer CPU station, communication between the control layer and the monitoring layer, and communication between the monitoring layer and the management layer. 4.2 DCS Scheme: This scheme utilizes DCS control technology, leveraging the remote I/O devices of the DCS to introduce signals into the main DCS, enabling centralized monitoring through a well-designed system. The advantage of this approach is that the same DCS can be used for all main equipment and auxiliary workshops, facilitating operation, maintenance, and spare parts management. For newly built power plants, this approach can be adopted if the performance-price ratio is acceptable. However, since new power plants often use island-specific bidding, with control instruments provided by different process equipment suppliers, the owner and design unit should coordinate with relevant suppliers as early as possible to resolve the selection of the DCS and the design of the control scheme. For older units, implementing this solution requires overhauling the existing control system, which presents challenges and requires cost considerations, as the control of various auxiliary systems may already employ PLCs or other solutions. However, some older power plants have adopted this approach. For instance, a power plant built two 200MW units in 1993 and 1994 used Xinhua's XDPS-400 distributed control system to form the plant's auxiliary DCS system. This system was divided into three subsystems based on the geographical location of the equipment: coal conveying, water treatment, and ash/slag. The DCS upgrades for the coal conveying and water treatment subsystems were implemented first. Optical cables were used to connect the two subsystems and the auxiliary control room, enabling integrated monitoring and resource data sharing between systems. The ash/slag subsystem utilized the existing (PLC + host computer) control system for expansion and improvement, ultimately integrating with the XDPS-400 system through a gateway to achieve data sharing and monitoring. Maintaining the principle of decentralized control equipment, the final goal was to achieve centralized monitoring of the three subsystems using three operator stations in the main control room of Unit #2. 4.3 PLC + Communication Network + Host Computer Solution For new units, a PLC of the same model can be connected to the DCS via a network, simplifying configuration, network setup, operation, maintenance, and spare parts management. However, for older units, since the PLCs used in different systems may vary and each system has its own host computer, considering both advanced technology and practicality, the existing host computer can be used as a gateway computer. After converting different communication protocols, it can be connected to an Ethernet network and then to the DCS and MIS, enabling monitoring through a computer in the control room. The main features of this solution are: the monitoring layer uses a PLC control network, which can better ensure real-time monitoring; the management layer uses a flexible computer network, facilitating network interconnection and system expansion. For example, the DCS auxiliary system of a power plant's 4×300MW units integrates seven independent systems, including ash removal control system, slag removal control system, condensate polishing control system, water supply, water purification, reverse osmosis, and coal conveying control system, into a single DCS system. This system is connected to the main control room, where the host computer monitors each system. The main control room of the unit has three monitoring host computers that are hot-standby for each other. One of them also serves as an engineering workstation. Each of the three host computers has an independent database, and the information comes from the PLCs of each auxiliary system. They are connected to the gateways of each auxiliary system through switches and optical cables, forming an Ethernet network that connects to the communication network of each auxiliary system, thereby enabling the monitoring of their respective PLCs. 4.4 The computer-based centralized monitoring system of the auxiliary system should have the following functions: (1) Complete process flow diagram display, real-time display of equipment status, control status and operating parameters. According to the specific requirements of the process and monitoring, a multi-layer display structure is adopted, including overview display, functional group display and equipment detail display. (2) Switching between different systems and screens and operation functions of control equipment. The selection of systems and equipment is convenient and simple, and there are operation permission settings and restrictions to meet the requirements of different operation levels. (3) Parameter alarm and accident recall function. (4) Printing function of historical parameters and curves, accident recall and operation records, alarms and reports. (5) Generation of the above functions and parameter modification function (i.e., engineer station function). (6) Reliable system self-diagnosis function so as to be able to diagnose the faults of system hardware and communication network in a timely manner. 5 Issues to be noted: (1) According to the overall requirements of the plant's automation level and the specific characteristics of the peripheral system's process, determine a reasonable and advanced overall monitoring scheme so that it can improve the plant's automation level and meet the needs of power plant management reform and development, and is feasible in combination with the actual characteristics of the site. (2) Ensure the safety and reliability of the specific scheme. The specific technical solution determined must first ensure the safety requirements of system operation, adopt redundancy technology, and design a complete self-test and alarm function to ensure the reliability of the overall network, power supply and PLC control. (3) The human-machine interface should be clear and easy to understand. Switching between main and auxiliary screens, between the overall view and detailed drawings, between various subsystems and between different process systems should be convenient and fast. There should be correct operation guidance and sequential control operation prompts, and operators should be able to monitor it easily. Important operations should have prompts and confirmations to avoid misoperation. (4) Debugging and maintenance interfaces should be reserved on site to facilitate on-site debugging and subsequent maintenance work. The system should also have a certain degree of scalability and online dynamic modification capability to adapt to future development and operation optimization of the auxiliary control system. (5) The on-site environment of the auxiliary system is relatively harsh. The selection of primary equipment and on-site installation and debugging work should be emphasized. Practice shows that the quality of primary equipment not only directly affects the installation and debugging of the control system, but also often becomes the main factor affecting the normal operation of the entire auxiliary system after the system is officially put into operation. (6) Prepare training for auxiliary system operators in advance so that they have a full understanding and operating experience of each independent auxiliary system, and cooperate with the effective operation guidance and prompts on the monitoring screen to ensure the correct operation of the system. 6 Conclusion (1) Realizing centralized computer monitoring of power plant auxiliary systems can reduce the number of posts and increase efficiency, improve the overall management level of the plant, increase labor productivity, and thus improve the competitiveness of the power plant, so that it is in a favorable position in the process of deepening power reform. (2) The rapid development of computer control and software and hardware technology, the continuous maturity of network technology, combined with the process and operation characteristics of power plant auxiliary systems, make it possible to realize computer monitoring of power plant auxiliary systems at this stage. (3) The detailed discussion of the overall scheme of centralized computer monitoring of auxiliary systems, the meticulous implementation of specific technical schemes, and the careful organization of installation and commissioning are the guarantee for the perfection of the system. (4) The effective division of labor in management and maintenance, and the advance training of operators are the guarantee for giving full play to the benefits of the system.